“Children who live in streets lined with trees are less likely to suffer from asthma”, the Daily Mail reported today. Several newspapers covered the research carried out in New York that found there to be fewer children with asthma living in neighbourhoods that had more trees. The Sun reported that the researchers found rates of asthma fell by a quarter when there were around 350 more trees in a square kilometre.

In this study, the researchers took into account various factors that could have affected the results, such as a neighbourhood’s proximity to pollution, relative wealth and population density. They were cautious in their interpretation, saying their study does not show that trees are causally related to asthma “at the individual level” (i.e. that trees may not have a direct effect on asthma symptoms, but instead could be markers of other environmental conditions that improve respiratory health for groups of people).

This is an important point and means there may be other individual factors, such as socioeconomic status that could be linked to both childhood asthma and the chances of living in a leafy neighbourhood.

Where did the story come from?

Dr Gina S. Lovasi and colleagues from Columbia University in the US carried out the research. The study was funded by the National Institute for Environmental Health Science and the Robert Wood Johnson Foundation. The study was published in the peer-reviewed Journal of Epidemiology and Community Health.

What kind of scientific study was this?

In this cross-sectional and ecological study, the researchers looked at how many cases of childhood asthma there were in 42 health service or hospital catchment areas in New York City. The rates of asthma prevalence were compared to the average density of trees in the areas the children lived in. The areas ranged from six to 67 square kilometres in size.

The researchers obtained information on asthma rates in four and five-year-old children from a 1999 school survey by the New York City Department of Health (NYCDOH). Information about the number of children under 15 who were admitted to hospital in 1997 was also obtained from the NYCDOH. This data was compared to the total number of children under 15 who lived in these catchment areas.

The density of trees on the streets of those areas was calculated from 1995 data provided by the New York Parks and Recreation department (the calculation used was the total number of trees on the parts of the streets within the hospital catchment area, divided by the size of the area). Other potential confounders (factors that the researchers thought might also be related to both tree density and asthma) were collected from data sources such as the 2000 census. These included the percentage of residents below a federal poverty line, ethnic mix and population density. They also measured how near the hospital catchment area was to sources of pollution such as major truck routes.

What were the results of the study?

The researchers report that “street tree density was high in the most densely populated areas and in areas with less poverty. Higher street tree density was associated with lower rates of childhood asthma even after [taking into account] potential confounders (including socio-demographic characteristics, population density, and proximity to pollution sources).”

There was an association found between density of street trees and rates of child hospitalisations, however, this association was no longer statistically significant (and therefore the result could have been caused by chance) once the researchers took the potential confounders into account.

What interpretations did the researchers draw from these results?

The researchers conclude that areas with more street trees experienced a lower prevalence of early childhood asthma.

They estimate that each increase in tree density of 343 trees per square kilometre is associated with a significant 29% lower prevalence of early childhood asthma.

What does the NHS Knowledge Service make of this study?

The design of this study means that it is not possible to conclude from the findings that planting trees would prevent childhood asthma for individuals living near them.

The researchers acknowledge this limitation by saying “the observational data may be subject to residual confounding or confounding by unmeasured characteristics”. By this, they mean that even though they took into account some socioeconomic factors, such as the percentage of people living below the poverty line, this may not have completely corrected any bias.

There are several other possible differences between people who live in leafy or non-leafy neighbourhoods that could account for the lower rates of asthma. For example, those who live in leafy areas could be better off financially, more likely to have health insurance and therefore better access to care. The researchers were unable to measure these at the population level as studies of data collected from individuals or houses would have been required.

As the researchers only reported the correlation between factors, but did not give the actual rates of asthma or the tree densities in the areas they looked at, it is not possible to judge how similar this US city is to a typical city in the UK or to gauge the importance of the 29% reduction in asthma rates.

There is also no information in the article on how the diagnosis of asthma was made (e.g. whether it was diagnosed by a doctor or if it was a parent reporting their child wheezing). There was also no information on the duration or severity of symptoms, its interference with daily life, or the need for medication.

Prevalence of asthma differs in various parts of the world and as this data was obtained in New York City, it cannot reliably be generalised to other urban areas where tree density, tree type or other types of environmental pollutants may differ.

The researchers call for others to repeat their study so that the link can be verified.

Sir Muir Gray adds...

Yet more evidence that green is good. We need an NHS forest, a million more trees around every health centre and hospital.